You have several options for iterating over a dictionary.
If you iterate over the dictionary itself (for team in league), you will be iterating over the keys of the dictionary. When looping with a for loop, the behavior will be the same whether you loop over the dict (league) itself, or league.keys():
for team in league.keys():
runs_scored, runs_allowed = map(float, league[team])
You can also iterate over both the keys and the values at once by iterating over league.items():
for team, runs in league.items():
runs_scored, runs_allowed = map(float, runs)
You can even perform your tuple unpacking while iterating:
for team, (runs_scored, runs_allowed) in league.items():
runs_scored = float(runs_scored)
runs_allowed = float(runs_allowed)